French patent application No. 2,316,026, filed on July 4, 1975 by the applicant and the INSTITUT DE MECANIQUE of Grenoble describes a device for confining a liquid metal jet. The device has a nozzle for forming the jet and is characterized by the fact that it comprises, in combination, means for creating an overpressure in the jet and means for discontinuing the overpressure. The means for creating the overpressure in the jet are formed by a coil surrounding the nozzle at the outlet thereof in combination with means for causing a high-frequency alternating current to pass through the coil.
The device in accordance with this patent application thus uses a combination of the following two means, for confining a jet of liquid metal
a coil which surrounds the nozzle through which the jet flows and which has a high-frequency alternating current flowing therethrough, this coil creating an overpressure in the jet at the outlet of the nozzle, and
means for discontinuing this overpressure.
It is stated in the above application that the frequency f of the alternating current which flows in the coil must be sufficiently high for the depth of penetration .delta. of the induction or magnetic field in the jet to comply with the following conditions:
.delta.&lt;R and PA1 .delta.&lt;e,
R being the radius of the metal jet before contraction and e the thickness of the metal screen. The following relations then hold: ##EQU1## with .sigma..sub.m and .sigma..sub.c representing the respective electrical conductivities of the metal which forms the jet (for example, steel or aluminium) and the metal which forms the screen (for example, copper) and .mu. designating the magnetic permeability of the liquid metal;
It is further stated in the above application that the means for discontinuing the overpressure in the jet is advantageously formed by a screen which is made from an electrically conducting material, is concentric with the coil and extends into the latter through the bottom (as considered in the direction of flow of the liquid metal). Means are provided for cooling the coil and the screen by removing the heat which is produced therein when the coil has an alternating current passing therethrough.
A french application related to the above-mentioned patent application, i.e. addition no. 2,396,612, filed on July 8, 1977 by the AGENCE NATIONALE DE VALORISATION DE LA RECHERCHE and the INSTITUT DE MECANIQUE of Grenoble, discloses means for regulating the flow of a liquid metal jet by using the device described in application no. 2,316,026. This is achieved by reducing the cross-section of the jet to a valve corresponding to the desired flow rate. Preferably the flow-regulating means are formed by means for varying the intensity of the high-frequency alternating current.
In both cases (original patent application and related application), means are provided for discontinuing the overpressure in the liquid metal jet. These means are advantageously formed by a cylindrical screen which is made from an electrically conducting material, is concentric with the coil and extends into the latter from the bottom (as considered in the direction of flow of the liquid metal jet).
U.S. Patent application no. 2,316,026 explains that the screen results in a sudden discontinuation of the magnetic induction and so discontinuation of the overpressure in the jet. This causes a reduction in cross-section of the jet because of the invariance of the flow rate Q=SV, on the one hand, and the pressure head (hydrostatic) ##EQU2## on the other hand, S being the cross-section of the jet, V the speed, P the pressure, and .rho. the density of the liquid metal, and g the acceleration of gravity. Thus, by reducing the pressure .rho. within the liquid metal jet, the flow speed V of the liquid metal is increased (because of the invariance of pressure head H) and the cross-section S (which is inversely proportional to the speed V because of the invariance of flow rate Q) is reduced.
The reduction in cross-section of the jet and accompanying separation of the jet from the nozzle takes place at the upper edge of the screen within the scope of the invention claimed in application Nos. 2,316,026 and 2,396,612.